1. Field of the Invention
The present invention relates to a telecommunications system, and more specifically to a transmission timing control apparatus that can be mounted on, or applicable to, communication nodes disposed in the form of, e.g. a sensor network, a mobile communications network or a local area network (LAN) to be spatially apart from each other.
2. Description of the Background Art
In order to allow such nodes spatially distributed to transmit data without colliding with each other, a time division multiple access (TDMA) system and a carrier sense multiple access (CSMA) system, such as a carrier sense multiple access with collision avoidance (CSMA/CA) system or a carrier sense multiple access with collision detection (CSMA/CD) system are available, as taught by Y. Matsushita et al. “Wireless LAN Architecture”, pp. 47, 53-59 and 69, Kyoritsu Shuppan Co., Ltd., Tokyo, Japan, 1996.
A problem with the TDMA system is that when an error occurs in a central control server the entire telecommunications system goes down. In light of this, there have been proposed various methods of allowing each of the nodes to mutually adjust its time slots by itself in a distributed coordination fashion without resorting to a central control server for thereby avoiding a collision between communication data. In accordance with such methods, each node periodically transmits and receives impulse signals with neighboring nodes for thereby mutually adjusting the allotment of time slots.
More specifically, each node uses a numerical formula modeling nonlinear oscillation to adjust the timing for transmitting an impulse signal in accordance with the timings at which the other nodes transmit input signals. Each node can therefore execute the adjustment in such a fashion that the timing for transmitting its own impulse signal is apart from the timings of the other nodes for transmitting impulse signals as far as possible, implementing the acquisition of time slots in a distributed coordination fashion.
When the communication control method stated above is applied to a radio or wireless communication environment, it is necessary to solve the problem of so-called hidden terminals. For that aim, the transmission output strength is controlled in such a manner that a transmitted timing signal in the form of impulse propagates over a range twice or more as broad as the propagation range of a data signal. For example, FIG. 2 shows the propagation ranges of data signals indicated by solid lines 31 and the propagation range of a transmitted timing signal indicated by a dotted line 33. By designing each of the propagation ranges 33 of timing signals transmitted from nodes N1, N2 and N3 twice or more as broad as the propagation ranges of a data signal, the nodes N1 and N3 are successfully prevented from simultaneously sending out data toward the node N2. In this manner, the nodes N1 through N4 interact with each other to control the data signal transmission timings thereof.
However, even if the transmission output strength is selected such that the propagation range 33 of a transmitted timing signal is twice or more as broad as the propagation range 31 of a data signal, it is likely that the former is not twice or more as board as the latter in an environment, e.g. where an electromagnetic wave sent from a node is attenuated by an obstacle. For example, as shown in FIG. 2, although the node N4 is able to receive data signals from both of the nodes N1 and N5, the node N5 stays outside the timing signal propagation range 31 of the node N1, and therefore unable to receive a timing signal sent from the node N1. It follows that the nodes N1 and N5 cannot interact with each other but are apt to send out data signals at the same time, resulting in a collision between the data signals.
Under the above circumstances, it is desirable to control the propagation range of transmitted timing signals and data transmissions in such a manner as to avoid a collision from occurring between data transmissions when the propagation range of a transmission timing signal is short of one two times longer than the propagation range of a data signal.